The invention relates to composite preform densification practices that use liquid infiltration to introduce ceramic matrix precursor into the void structure of the preform and concerns a method for keeping the liquid in the void space prior to rigidization, thereby completely filling the void space.
Manufacturing methods for several useful types of fiber reinforced ceramic matrix composites start with construction of a dry yarn preform; this is followed by construction of the ceramic matrix within the void spaces among the fibers of each yarn and among the yarns of the preform. Matrix construction is commonly accomplished through one or more densification cycles comprising introduction into the void spaces of matrix precursor material in either the liquid or gas phase. For liquid densified carbon-carbon and other composites intended for use as refractory materials, the desired level of gross matrix density and continuity necessitates repetition of the densification process several times, among which are interspersed very high temperature process steps. These steps pyrolyze the matrix precursor, thereby synthesizing carbon or ceramic material as volatile constituents and decomposition products are driven off. Pyrolysis also creates additional void space within the partially constructed matrix, which is substantially filled by subsequent densification cycles.
An adequate bond between the fiber and matrix fractions, essential to proper behavior of the composite material, must be achieved during densification. Thus the viscosity of the liquid precursor of the ceramic matrix must be low enough to enable penetration of the yarn and to assure wetting of the fibers. Further, the liquid precursor must have a char yield (residual weight after pyrolysis to carbon or ceramic material) sufficient to substantially preserve the fiber to matrix bond and to minimize the number of densification cycles needed to attain the desired matrix density.
An alternative to liquid densification is chemical vapor infiltration or deposition (CVI or CVD), commonly carried out at substantially elevated temperatures when the end product is a refractory. Among the important features of the CVI/CVD process are facile skinning of the component surface, a surface-to-core gradient of decreasing density, and substantial cost of the facility needed to perform CVI. Where the end products are flat laminates, such as carbon-carbon brake shoes, the reflection of the facility cost in the unit cost of product is typically minimized by substantially filling the working volume of the facility with product. For end product with large bulk or tubular form, the facility cost is a formidable obstacle to the use of CVI/CVD densification.
The invention herein is an enhanced version of a common process for the densification of composite material involving the infiltration of a thermosetting liquid resin such as phenol formaldehyde resin (resole and novolac) at or near room temperature or a thermoplastic such as moltein pitch into porous preforms with various fiber architectures made from refractory fibers such as carbon, graphite, silicon carbide, mullite (aluminum borosilicate), glass, alumina, silica, quartz, etc. Densification in the normal way proceeds by causing the liquid to flow into the interbundle and intrabundle voids by pressurization, evacuation prior to infiltration, or both.
Resin infiltration is often followed by removal of the infiltrated preform from the resin bath, so as to eliminate or minimize the need, during and after the densification, to machine the product to final form, and so as to preserve the excess resin remaining in the bath for later reuse. Removal from the bath enables a partial loss of resin from the product, by flow out of the larger voids due to the low viscosity essential to achieving the necessary penetration. The resin is next rigidized by cure at moderately elevated temperature, and is then pyrolyzed at much higher temperature. When the resin is phenol formaldehyde, the cure reactions liberate water, resulting in additional void space within the matrix.
As alternatives to early removal from the resin bath, the resin may be advanced in cure while the preform remains in the resin bath, so that removal is delayed until a high enough viscosity is achieved to preclude run out, or until the resin is completely cured.
Thermoplastics such as pitch must be melted prior to infiltration. If the product is cooled for rigidization after infiltration, the pitch on the exterior of the preform is then removed and reused. In the subsequent pyrolysis and/or carbonization, the pitch in the product must pass again through its melting point and in so doing, melts and partially flows out of the part.
Retention of a thermoplastic such as pitch in the preform can be increased by processing the preform while inside a block of pitch held in a process container to prevent resin flow out during carbonization. Alternatively, the loss of pitch during heating at the outset of the pyrolysis step can be diminished by packing furnace black filler around the product, within a processing container. In both cases, the process wastes matrix precursor material, requires the introduction of operations to remove the excess, and produces volatiles and solid residues that demand control and disposal in an environmentally safe manner.
For further background, see U.S. Pat. No. 4,364,879 which is hereby incorporated by reference, in toto.
The densification process as described above is enhanced by use of an actinic radiation curable thermoset resin as a barrier material. The actinic radiation cured resin would replace the normal class of phenolic resin derivatives in at least one of the densification cycles. With the actinic radiation cured resin, such as a low viscosity high char yield modified epoxy novolac, a densification cycle is started conventionally by causing the resin to infiltrate the porous preform with or without either the aid of prior evacuation of atmosphere or the subsequent application of pressure during infiltration or both. An important feature of the invention is to apply the actinic radiation to the infiltrated preform during or immediately after withdrawal from the resin bath to advance the resin at the surface of the preform so that the amount of resin left in the part prior to cure can be maximized while using the minimum amount of resin, thereby improving the weight pickup per densification cycle. During the removal process, the resin bath may be shielded from the actinic radiation to minimize the incidence of actinic radiation on the surface of the resin bath. At worst, a skin would form on the resin bath after removal of the part; the skin at the top of the resin bath, readily stripped off and discarded, constitutes the only unused resin lost to further reuse.
The low viscosity high char yield modified epoxy novolac resin offers the further advantage of liberating little or no water during its cure, thus minimizing this source of matrix void space upon pyrolysis.
In the case of densification using a thermoplastic such as pitch, the infiltrated preform would first be removed from the cooled block of solidified pitch. Second, the excess pitch would be removed for reuse and finally the surface of the infiltrated preform would be coated with actinic radiation curable resin such as high char yield modified epoxy novolac, in this application, preferably formulated to have high viscosity, and would be cured to make a hard barrier that retains its rigidity through carbonization. This has been recently demonstrated.
This invention is a method to densify a refractory fiber preform which comprises impregnating the preform with a major portion of a liquid, ceramic-forming precursor with actinic radiation thereby forming a sealing barrier or skin on the surface of the preform. Preferably, the liquid precursor is a high char yield low viscosity epoxy novolac resin modified by the introduction of an additive to effect actinic curing. By actinic curing is meant curing initiated by electromagnetic radiation of all kinds as well as electron beam and other particle beam radiation. The preferred radiation is ultraviolet. Also preferably the curing is initiated immediately after the impregnation, so that when impregnation is by means of immersion in a bath, cure is initiated by actinic radiation during or soon after the withdrawal of the preform from the bath of the liquid precursor. Cure completion may be accomplished by actinic radiation or, preferably, thermally since penetration of the radiation into the inner portions of the preform could be difficult. This invention magnifies the effectiveness of the liquid impregnation method of matrix construction by enabling the filling of void spaces within the fiber preform with matrix precursor rather than merely coating their interior surfaces, thereby diminishing the number of impregnation cycles necessary to create the desired matrix gross density and continuity. This method is also the use of the preferred resins for two cases: 1) liquid infiltration with rigidization during component removal from an infiltration bath and 2) enhancement of the char yield of liquid infiltrant where the infiltrant is frozen by allowing the temperature to fall to room temperature, the excess solidified infiltrant is machined or chipped off, and the surface is coated with an actinic curable resin to provide a barrier to the flow of the infiltrant out of the component before it can be charred in place. For case 1, this method is also the use of a resin and an additive; the resin may be selected from the group consisting of epoxy-functionalized phenolic resins such as DEN 431, DEN 438 and DEN 439 manufactured by Dow Chemical U.S.A.; or EPN 1138 and EPN 1139, manufactured by Ciba-Geigy; epoxy-functionalized cresol such as Ciba-Geigy ECN 1235 with reactive diluents such as Union Carbide""s Cyracure UVR-6200; and epoxy-functionalized bisphenol A resins such as Ciba-Geigy""s GT 6060; the additive may be selected from the group consisting of ultraviolet curing agents such as triarylsulfonium hexafluoroantimonate (Union Carbide""s Cyracure UVI-6974) and methacrylates with or without the concurrent use of aliphatic amines such as hexamethylenetetramine; aromatic amines such as benzyl dimethyl amine (Ciba-Geigy""s DY 062); heterocyclic amines such as Ciba-Geigy""s DY 070; amine complexes such as boron trichloride-amine complex (Ciba-Geigy""s DY 9577) and amine-phenol complex (Ciba-Geigy""s DY 073); anhydrides such as nadic methyl anhydride and hexahydrophthalic anhydride; or anilines such as methylene dianiline where reactive diluents such as Cyracure UVR-6200 are used as required to modify the viscosity of the formulation. The preferred combination is DEN 431 epoxy novolac resin with Cyracure UVI-6974 additive. For case 2, this method is also the use of the preferred impregnant selected from the group consisting of thermoplastics such as pitch, polycarbosilane, and polybenzimidazole and a barrier material selected from the group of actinic curable resins stated for case 1.
The irradiated resin retains its rigidity while thermally decomposing hence it can perform as a barrier through the cure rigidization occurring in case 1 and through the pyrolysis of the impregnant in case 2. When used either as an impregnant or as a coating over a thermoplastic impregnated preform, the actinic cured resin acts as a surface barrier preventing loss of liquid from the interior of the preform. The invention maximizes the amount of densification per cycle while minimizing the amount of resin used thereby minimizing environmental control and related process costs.